The present invention relates to a process for decomposing an aromatic acylated compound.sup.. HF-BF.sub.3 complex in preparing an aromatic acylated compound by decomposing an aromatic acylated compound.sup.. HF-BF.sub.3 complex obtained by reacting an aromatic compound with an acylating agent in the presence of HF-BF.sub.3 used as a catalyst.
It is already known to obtain an aromatic acylated compound by reacting an aromatic compound with an acylating agent in the presence of HF-BF.sub.3 as a catalyst. For example, Japanese Patent Application Kokai (Laid-open) No. 54-135756 discloses a method of obtaining a 2-alkyl-6-acylnaphthalene by reacting a 2-alkylnaphthalene with an acylating agent in the presence of HF and BF.sub.3. EP-A-0215351 discloses a method which comprises first converting an acid anhydride into an acyl fluoride in an acylating agent synthesis step, and then obtaining an aromatic acylated compound in the presence of HF and BF.sub.3 by using the isolated acylfluoride in an acylated compound synthesis step.
The reaction produced liquids obtained by these methods are each a HF solution of an aromatic acylated compound.sup.. HF-BF.sub.3 complex. On heating the liquid, the linkage between the aromatic acylated compound and HF-BF.sub.3 is decomposed and HF and BF.sub.3 are recycled for use as catalyst.
For separation of the aromatic acylated compound and HF and BF.sub.3 from the reaction produced liquid, the following methods have hitherto been proposed:
(1) a method comprising heating the liquid at 20.degree.-40.degree. C. under reduced pressure to decompose the aromatic acylated compound.sup.. HF-BF.sub.3 complex and distilling HF and BF.sub.3 away in the form of gas to effect separation,
(2) a method comprising adding to the liquid a nitrogen compound such as ammonia, monoethylamine, piperidine, acetonitrile, nitroaniline, and chloroaniline to form a molecular compound, separating the molecular compound, and then heating it or reacting it with sulfuric acid, etc. to separate HF and BF.sub.3.
(3) a method comprising adding a decomposing agent inert to HF and BF.sub.3, e.g. aromatic hydrocarbons or halogenated aromatic hydrocarbons such as benzene, toluene and chlorobenzene (these aromatic hydrocarbons are hereinafter referred to as A.H.) and heating the mixture under reflux of the agent to effect decomposition.
The aromatic acylated compound in the reaction produced liquid obtained by reacting an aromatic compound with an acylating agent using HF-BF.sub.3 as catalyst is susceptible to deterioration by heating, so that the above-mentioned operations need to be carried out as rapidly as possible. The prior methods for decomposing aromatic acylated compound.sup.. HF-BF.sub.3 complex have the following difficulties.
(1) In the method of heating under reduced pressure, the degree of pressure reduction is determined according to the boiling point of acetylated compound. Aliphatic acylated compounds such as acetyl fluoride, propionyl fluoride and isobutyryl fluoride have relatively low boiling points, so that HF-BF.sub.3 can be separated at temperatures of 20.degree.-40.degree. C. under low degree of pressure reduction. However, since aromatic acylated compounds have high boiling points, a considerably high degree of vacuum (namely, degree of pressure reduction) is necessary to distil the compounds at said temperature, which requires much power. Further, HF and BF.sub.3 are highly corrosive and can incur great danger if air leaks into the apparatus under high vacuum. Therefore, this method is difficult to practice on commercial scale.
(2) The method comprising adding nitrogen compounds to form molecular compounds and then decomposing the latter compounds newly requires operations of separating the molecular compounds. Accordingly, the catalyst recovery step becomes very complicated and the method is not suited to practical use.
(3) In the method of using inert decomposing agents, usually a decomposition-distillation column provided with a heater at the column bottom is used to effect the separation of catalyst as rapidly as possible. The inert agents used are the above-mentioned A.H. The aromatic acylated compound.sup.. HF BF.sub.3 complex is rapidly decomposed by contact with the A.H. vapor and is distilled, while HF and BF.sub.3 are separated from the column top. However, a part of A.H. is entrained with HF and BF.sub.3 and is distilled out from the column top along with HF and BF.sub.3. HF and BF.sub.3 are cooled in a condenser together with A.H. vapor, HF and A.H. are condensed, and BF.sub.3 is separated as non-condensable gas. The condensate is separated in a separator into HF and A.H.. The separated HF contains A.H. associated with the non-condensable gas (BF.sub.3), dissolved therein. When the concentration of A.H. in HF is high, alkylated products and acylated products of said A.H. are formed by side reactions in the acylating agent synthesis step using recovered HF, resulting in decrease in yields in the acylating agent synthesis step and in the synthesis step of intended acylated compounds. Further, since the alkylated products of A.H., e.g. tetraisopropylbenzene, is insoluble in HF, they can cause blockages of piping, etc. Thus, the A.H. becomes an obstacle to the recycling of HF and BF.sub.3 of the catalyst. Although there is further known, to separate A.H. dissolved in HF, a method comprising subjecting the HF solution after separation of non-condensable gas (BF.sub.3) to redistillation, this method requires a separate distillation column which requires a considerably large number of stages and reflux ratio. Therefore, the construction cost becomes high and energy consumption for redistillation is large.